Questioning WeakMap.prototype.clear

Andrea Giammarchi andrea.giammarchi at
Tue Jan 22 23:45:42 PST 2013

I would like to humbly say that if this `WeakMap#clear()` method is anyhow
slowing down or reminding the adoption of the WeakMap concept itself for
all browsers, just mark it TBD and go on with the rest: WeakMap is needed
and wanted since ever, `clear()` gotchas can be discussed for all the time
we need to make it right, it should not stop or block WeakMap
specifications for ES6, a big IMHO in this reply and thanks for reading
'till here.

On Tue, Jan 22, 2013 at 11:27 PM, Tom Van Cutsem < at> wrote:

> I've been following the threads on WeakMap.prototype.clear with interest,
> and I think I may have an argument that should make the existence of
> WeakMap.prototype.clear irrelevant from an ocap security perspective.
> It was already previously suggested in this thread that the ocap thing to
> do would be to prevent access from untrusted code to your WeakMap instance
> in the first place.
> I think the suggestion that WeakMaps could be shared freely safely if only
> they didn't have a .clear() method is wrong.
> Consider for a moment that WeakMaps don't have a .clear() method so you
> know no one can get at your values without having access to the
> corresponding keys. So you share your WeakMap freely with untrusted code.
> This still leaves you open to another attack. Before I can describe the
> attack, I think it's useful if people have some understanding of what the
> "rights amplification" pattern actually is.
> The "rights amplification" pattern that MarkM previously referred to is
> often used to convert an opaque, untrusted object reference into a value
> your code can trust.
> In my trusted code, I can write:
> token = Object.freeze({});
> myWeakMap.set(token, myTrustedObject);
> return token;
> I now pass around the token to code in the wild.
> The token by itself is useless. It's only useful if someone later passes
> it back to my code:
> function doSomethingWith(token) {
>   // token could be anything, I don't trust it
>   var trustedObj = myWeakMap.get(token);
>   // if trustedObj is not undefined, the token was legitimate
> }
> This pattern really hinges on the WeakMap being properly encapsulated. If
> it isn't, an attacker could just mint his own tokens, add them to the
> WeakMap, and then fool my trusted code into thinking that the token is
> legitimate.
> In other words: if one wants to share a WeakMap with untrusted code, one
> must already wrap it such that access to the "set" method is prohibited as
> well. In this regard, also having to remove .clear() is a marginal extra
> cost, compared to the more general utility of this method when using
> WeakMaps for caches rather than for rights amplification.
> Cheers,
> Tom
> 2013/1/22 David Bruant <bruant.d at>
>> Le 22/01/2013 15:59, Jason Orendorff a écrit :
>>> A) Are there more WeakMap applications that will want .clear() or
>>> applications that will want .clear() not to exist? Offhand I would bet on
>>> the former, by a landslide, but if you think otherwise, or if there's some
>>> other reason to privilege .clear() not existing, let's talk about that.
>>> (...)
>>> Having said all that, I bet we could hack around the worst-case GC
>>> performance. It'll be a pain, but GC is like that sometimes. This decision
>>> should hinge on what provides the best API for developers. I think we
>>> mainly disagree on what developers want, which is a great thing to talk
>>> about. Let's talk about that.
>> I agree use case dominant is a crucially important answer. To date, I
>> honestly don't know which case of want-clear and don't-want-clear would be
>> dominant. I agree Allen showed a compelling use case but I can't judge how
>> important it is. In my experience, I've not had the need for a .clear yet.
>> I know that in some occurences, my code relies on the weakmap not being
>> emptied, but I've kept the weakmap well-encapsulated in these cases so this
>> experience is not that relevant (because intrusive .clear can't happen).
>> I've written a whole paragraph which is the most factual I can say on the
>> topic, but doesn't help the debate much.
>> When it comes to feelings, I prefer prudence by default. I'd like to say
>> a few words about that. I understand that features shouldn't be seen only
>> with ocaps eyes, but I'd like to take a moment to describe what I care
>> about when it comes to ocaps and what we call "security".
>> Node.js is an interesting ecosystem. There are a lot of modules, it's not
>> unusual to use 10-20 modules in a project which can make 100+ modules when
>> counting recursively. Because it costs a lot of time, it's not possible to
>> rewrite everything, it's not possible to contribute the necessary test
>> coverage to modules and it's not possible to do careful security reviews of
>> all used modules (and updates!).
>> However, it's possible to apply POLA (Principle Of Least Authority), that
>> is give to each module the information and capabilities it needs to do its
>> job and no more. If WeakMap.prototype.clear gets natively in the language,
>> it means *all* modules can have an irrevocable right to flush any weakmap I
>> hand them.
>> It's the same sort of problem than if a "free" operator was brought to
>> JavaScript (in advance, I agree that the .clear is more acceptable) as
>> suggested once [1] (almost ironically by Node's lead?). Suddenly, a module
>> could free objects you hand out to them. A module thinks it's freeing one
>> of it's own objects but actually frees one of yours because of a bug? Too
>> bad, you'll be throwing a TypeError very soon and who knows in which state
>> it will leave your application.
>> Dave Herman made an equivalent case about coroutines [2]. It provides
>> abusive authority: you call a module-imported function and for whatever
>> good or bad reason, it can suddenly stop the stack. It makes your code
>> harder to reason about because when you wrote the code, you probably
>> expected the function call to return (or throw).
>> Back to weakmaps, the issue here is not technical, but... cultural I
>> would say. I can decide to encapsulate my weakmap in a WeakMapWithoutClear,
>> but doing so, I cut myself from modules which take .clear for granted. A
>> module does rely on WeakMap clearbility? It will hand me the weakmap I'm
>> supposed to use and I know in advance anything can happen, because I didn't
>> create this object.
>> If weakmaps don't have a clear, modules using language weakmaps won't
>> take it for granted and you can be fearless about sharing your weakmap...
>> very much like you can be  hand objects today without the fear of them
>> being free'd by mistake or malice using a free operator.
>> Since I'm on the topic of language-based abusive authority, I've come
>> across a case where a logging library would throw at my face [3] anytime it
>> would have to log about an object with a cycle in it [4]. Logging libraries
>> are really not the one you'd expect to throw an error so we didn't wrap the
>> call in a try-catch. So logging some objects would make the application
>> crash.
>> I have come to think that error-throwing with stack-unwinding is an
>> abusive authority. Especially given that try-catch is an opt-in.
>> I understand now that silently swallowing errors is not a good idea and
>> the need to report errors in a different channel than return value, but I
>> don't think stack-unwinding is a good default for that. Promises show an
>> interesting model; to keep in mind for another language maybe.
>> The point I tried to make here is that POLA allows to build applications
>> using untrusted modules without fearing them and that's really an excellent
>> property, because we use constantly huge amounts code we don't necessarily
>> trust or have the time to review or test besides what we test during the
>> development period.
>> WeakMap.prototype.clear is at a different scale of "danger" and abusive
>> authority than a hypothetical free operator or coroutine or abusive
>> stack-unwinding, but it's an issue of the same family.
>> Whether the smaller scale of danger makes it acceptable is a good
>> question. I would answer "no" by default, but I can understand if others
>> say "yes" for this one. To be balanced with advantages that native
>> WeakMap.prototype.clear provide which are yet to be accurately determined.
>> David
>> [1]**pipermail/es-discuss/2012-**
>> October/026007.html<>
>> [2]**2011/12/14/why-coroutines-**
>> wont-work-on-the-web/<>
>> [3]**winston/issues/151<>
>> [4]**winston/issues/100<>
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